In the manufacture of paper pulp, it is important to know the degree of fibre concentration in the paper pulp, as this determines the characteristics of the finished product. It is therefore customary, in a refiner designed for the manufacture of paper pulp, to utilize optical concentration meters for this purpose, arranged in a blow line coming out from the refiner.
Such a method is described in WO 86/07458, in which at least two optical concentration meters are used which shine through the finished paper pulp with infrared and polarized light respectively, and thereafter measure the absorption and reflection of the light and the de-polarizing effect respectively. The measurement results are then used to calculate the degree of fibre concentration in the paper pulp.
U.S. Pat. No. 4,171,916 also describes a method in which polarization, absorption and reflection of the transmitted light are used to determine the degree of fibre concentration in the paper pulp.
These methods work relatively well provided that the degree of fibre concentration is not too high, preferably below approximately 5% by volume, but are less applicable for paper pulps with a higher degree of fibre concentration. As, in certain cases, it is desirable to achieve a degree of fibre concentration of upwards of 30% by volume, it is desirable to produce a method and a device that can produce results that are reliable even at such high degrees of fibre concentration.